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LASER Introduction
The trouble with a subject like Laser light is where to start describing how it works. Perhaps a little
history lesson might be the best place. The most famous of those who first postulated the idea of a
laser was non-other than Albert Einstein himself back in the 1920s and 1930s. At that time light was
considered to be what is called spontaneous light.
This spontaneous light is emitted by hot objects or by
individual atoms in an electrical discharge. Each particle
of this light, referred to as a photon, is emitted in a
random fashion without any commonality with any of
the other particles being emitted in the same vicinity. It is
given this name, Spontaneous Light, because each photon
acts in an independent and unpredictable way and its
creation is known as Spontaneous Emission of Radiation.
Einstein and other researchers however predicated the
possibility of another type of light that they referred to as
stimulated light.
This was based on the idea that an excited atom could
Different Photons
produce an exact copy of a passing photon. Unlike the
spontaneous light, where nearby photons were all different, photons
produced by this Stimulated Emission of Radiation was so similar
that they were indistinguishable from each other. We will see that
Laser light is formed from this
type of stimulated emission of
radiation, in fact that is part of
what the acronym LASER
stands for.
Spontaneous or incoherent Light
Stimulated or Coherent Light
Light Amplification for Stimulated Emission of Radiation.
So how does an atom get excited and how does an excited atom produce a photon?
We can get an atom excited by giving it some energy. This energy allows the electrons (particularly the
outer electrons) to move further away from the nucleus of the atom. This is not their normal state, so
before long the atom will return to its normal (or ground) state, by the electrons releasing the energy
they were given. This energy is released as a photon of light. This release is spontaneous (i.e. Not
triggered or stimulated by something else) and occurs by one of the electrons accelerating back and
fourth and then emitting the photon. The type of photon and direction is random as it happens
spontaneously.
However if an excited atom is getting ready to release, as another photon is passing by, then the passing
photon can trigger (stimulate) the atom to release a photon that is identical to the photon that triggered
it. This is caused by a sympathetic vibration being passed from the passing photon to the electron in
the excited atom, that starts to accelerate back and fourth at this same vibration frequency and causes a
photon with the exact same properties as the original.
When this idea was first discovered it was immediately recognized that his could allow for the
possibility of a process called Light Amplification. If enough excited atoms could be put together then
a passing photon could encourage the production of duplicate photons, thousands even millions or
billions of them.
Of course this was all theoretical, it was not until the 1950s that the detail of how to go about light
amplification were finally worked out. It would be the 1960s before the first laser oscillatiors were
actually constructed. These devices could emit an intense beam of light made up of photons which
were all exact copies of each other. This meant that together they formed a single electro-magnetic
wave. This is known as coherent light. One of the special properties of this coherent light is that it is
all at a single frequency. In terms of light, its frequency is its colour. So it is mono-chromatic light, i.e.
light that has only one colour. This will prove very useful in certain applications.
So how do we achieve light amplification?
To produce coherent light you must start with one photon and duplicate it many times. So when a
photon, which on its own is not very bright, enters a laser amplifier, which is an appropriate collection
of excited atoms then the light is amplified and becomes brighter because it leaves as many photons of
light, all identical of course.
A Laser oscillator is a device that encloses the laser medium between two specially designed mirrors.
Inside the laser medium many of the excited atoms will spontaneously produce photons. If one of
these photons happens to meet the specific design of the Laser system it will bounce of one of the
mirrors and return in the exact direction of the other mirror. When it reaches the other mirror it will
reflect back in the direction of the first mirror. This will continue indefinitely. However with each pass
through the laser medium it will stimulate the emission of photons from excited atoms, and these
photons will be exact duplicates of itself. As they are exact duplicates they will travel in the same
Laser Medium, with Mirrors
direction and the same bouncing back and fourth off of each of the mirrors will happen to them also.
Of course these new photons will themselves stimulate duplicates which will follow the same process.
Now if both mirrors were perfectly reflective, this process would be a bit pointless as there would be
lots of these photons making a coherent light, but none could escape. Instead one of the mirrors is
semi-transparent. This allows some of the photons that hit this mirror to escape. It is this escaping
light that creates the usable laser light. However if we let the light escape, then eventually all of the
excited atoms will have given up their energy and no more photons will be produced. So it is important
that the atoms are re-excited by giving them energy. This is called pumping the laser. How the
pumping is achieved is dependant on the type of laser, but it is normally achieved by passing an
electrical current through the system or intense light is shone on the laser medium. Of course this
intense light is incoherent light caused by spontaneous emission i.e. normal light.
The light that escapes through the semi-transparent mirror is called a laser beam and it consists of
duplicates of the one original photon that set it all off. Since virtually all of the photons in a laser
beam are identical, they can all focus together to an extremely small spot.
Laser Beam
Classes of Laser
Class 1
CLASS 1 LASER PRODUCT
A class 1 laser is safe under all conditions of normal use. This means the maximum permissible exposure (MPE) cannot be exceeded.
Class 1M
LASER RADIATION DO NOT VIEW DIRECTLY WITH OPTICAL INSTRUMENTS
CLASS 1M LASER PRODUCT
A Class 1M laser is safe for all conditions of use except when passed through magnifying optics such as microscopes and telescopes. Class 1M lasers
produce large-diameter beams, or beams that are divergent. The MPE for a Class 1M laser cannot normally be exceeded unless focusing or imaging optics
are used to narrow the beam. If the beam is refocused, the hazard of Class 1M lasers may be increased and the product class may be changed. A laser can
be classified as Class 1M if the total output power is below class 3B but the power that can pass through the pupil of the eye is within Class 1.
Class 2
LASER RADIATION DO NOT STARE INTO BEAM
CLASS 2 LASER PRODUCT
A Class 2 laser is safe because the blink reflex will limit the exposure to no more than 0.25 seconds. It only applies to visible-light lasers (400–700nm).
Class-2 lasers are limited to 1mW continuous wave, or more if the emission time is less than 0.25 seconds or if the light is not spatially coherent.
Intentional suppression of the blink reflex could lead to eye injury. Many laser pointers and measuring instruments are class 2.
Class 2M
LASER RADIATION DO NOT STARE INTO BEAM OR VIEW DIRECTLY WITH OPTICAL INSTRUMENTS
CLASS 2M LASER PRODUCT
A Class 2M laser is safe because of the blink reflex if not viewed through optical instruments. As with class 1M, this applies to laser beams with a large
diameter or large divergence, for which the amount of light passing through the pupil cannot exceed the limits for class 2.
Class 3R
LASER RADIATION AVOID DIRECT EYE EXPOSURE
CLASS 3R LASER PRODUCT
A Class 3R laser is considered safe if handled carefully, with restricted beam viewing. With a class 3R laser, the MPE can be exceeded, but with a low risk
of injury. Visible continuous lasers in Class 3R are limited to 5 mW. For other wavelengths and for pulsed lasers, other limits apply.
Class 3B
LASER RADIATION AVOID EXPOSURE TO BEAM
CLASS 3B LASER PRODUCT
A Class 3B laser is hazardous if the eye is exposed directly, but diffuse reflections such as from paper or other matte surfaces are not harmful. Continuous
lasers in the wavelength range from 315 nm to far infrared are limited to 0.5 W. For pulsed lasers between 400 and 700 nm, the limit is 30 mW. Other limits
apply to other wavelengths and to ultrashort pulsed lasers. Protective eyewear is typically required where direct viewing of a class 3B laser beam may occur.
Class-3B lasers must be equipped with a key switch and a safety interlock.
Class 4
LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION
CLASS 4 LASER PRODUCT
Class 4 lasers include all lasers with beam power greater than class 3B. By definition, a class-4 laser can burn the skin, in addition to potentially devastating
and permanent eye damage as a result of direct or diffuse beam viewing. These lasers may ignite combustible materials, and thus may represent a fire risk.
Class 4 lasers must be equipped with a key switch and a safety interlock. Most industrial, scientific, military, and medical lasers are in this category.
Applications of LASERs
Optical Disc Systems. CD, DVD and Blu-Ray.
It was Philips Electronics that first set to work on designing the Compact disc player. This was meant
to be a digital optical disc of a similar size to the compact cassette tape that had been so popular. What
they wanted was a contactless technology. This would mean the sensor that read the music data from
the disc would not actually touch it and therefore there would be no wear and tear. Wear and tear on
Vinyl records and audio cassette caused an on going degradation of the sound.
But when they first went to design the CD system it was a very different world to today. For starters
they knew they would require a laser. A laser that could do the job would cost thousands and would be
very large. It would also likely need a specialist technician to operate it. When you work for a large
company like Philips though you usually know what innovations are happening elsewhere.
One such innovation was the semi-conductor laser (laser diode). This could be made very small and
from very inexpensive materials, but at the time had a life span of only a few seconds. Philips went
ahead with the design anyway in the hope that by the time they were ready, the laser diode would be
ready for production and robust enough for use. Luckily this paid off and the laser diode was ready in
time for the introduction of the CD.
Another reason the Laser was so useful for the CD player was that it allowed for very inexpensive
optics.
The laser light would need to be focused to see the data on the CD, this required optics. One of the
problems with low quality optics is that when light enters it, dispersion takes place.
When a light wave arrives at an interface with a denser medium, such as the interface between air and
the surface of an optical disc or a glass lens, the velocity of the wave is reduced; therefore the
wavelength in the medium becomes shorter, causing the wave to leave the interface at a different angle.
This is known as refraction. The ratio of velocity in a
vacuum to velocity in the medium is known as the
refractive index of that medium; it determines the
relationship between the angles of the incident and
refracted waves. If the speed of light in the medium
varies with wavelength, then dispersion takes place.
Dispersion is where light gets separated out into its
different wavelengths (frequencies, colours) e.g. You see
this when sunlight reflects off water particles and is
separated into the colours of the rainbow. This can cause
a lack of precision when trying to read the data on a CD.
Luckily though because Laser light is mono-chromatic it
only has one frequency/wavelength of light and therefore
dispersion cannot occur. These inexpensive optics meant
that the CD lens system would not add much to the price
of the machine and would thus make it available to the
ordinary consumer.
In fact one of the interesting things about the CD is that
they actually decided to leave some of the focusing of the
light to the plastic of the CD itself. This wouldn't have
worked if it wasn't mono-chromatic laser light as
dispersion in the plastic of the CD would have ruined the
Laser Light passing through CD
preciseness of the focus. Just look at a CD yourself in
any normal light and you will see the different colours of
the rainbow in it.
So let’s take a look at how the laser focuses on the CD.
In trying to make the CD a universal disc for the masses one of the goals was that no special working
environment or handling skill is required. However there is still a need to record the information at a
very high density to keep the size of disc small for a reasonable playing time.
High-density recording means the data on the disc is very small and to be seen precisely this requires a
light of a short-wavelength.
The information layer of a CD is read through the thickness of the disc. The diagram above shows
that this approach causes the readout beam to enter and leave the disc surface through the largest
possible area. The actual dimensions involved are shown in the figure. Despite the minute spot size of
about 1.2 μm diameter, light enters and leaves through a 0.7 mm-diameter circle. As a result, surface
debris such as dust or scratches must be almost 600 times larger than the readout spot before the beam
is completely obscured. You should note that because the data on the CD is actually much closer to the
label side, then a scratch on that side can actually remove the data. Despite this, we for some reason
tend to place CDs label side down on surfaces in the belief they will be less likely to get damaged this
way.
Try a test with a Recordable CD that you don't mind destroying: On the label side mark it with a sharp knife or
write on it with a ball point pen then hold it up to the light. As likely as not you will have removed the data and you will
be able to see right through the plastic of the disk.
Some of the light reflected back from the disc re-enters the aperture of the objective lens. The pickup
must be capable of separating the reflected light from the incident light. The diagram below shows
two systems which can be used to do this. In (a) an intensity beam-splitter a semi-silvered mirror is
inserted in the optical path and reflects some of the returning light into the photosensor. This is not
very efficient, as half of the replay signal is lost by transmission straight on. In the example (b)
separation is by use of a polarising prism.
In natural light, the electric-field component will be in many planes. Light is said to be polarised when
the electric field direction is constrained to one plane only. A device known as a quarter wave plate can
be used to rotate the plane of polarisation. Rotation of the plane of polarisation is a useful method of
Using a Semi-silvered mirror to separate the
incident light and the reflected light
separating incident light (coming from the Laser)
Using a Polarising Prism to separate the
incident light and the reflected light
and reflected light in a laser pickup. Using a quarter-wave plate, the plane of polarisation of light
leaving the pickup will have been turned 45o, and on return it will be rotated a further 45o, so that it is
now at right angles to the plane of polarisation of light from the source. The two can easily be
separated now by a polarising prism, which acts as a transparent block to light in one plane, but as a
prism to light in the other plane, such that reflected light is directed towards the sensor.
The frequency of laser light used for the CD is in the
infra-red range. This is at a slightly longer wavelength to
the red light that the Human eye can see. In order fit
more information to a disk the data would have to be
made smaller. To precisely see these smaller bits of data
you need a shorter wavelength (i.e. higher frequency).
So when DVD was being designed, a red laser was
required that had a shorter wavelength, as the data was
so much smaller than on the CD in order to pack in so
much of it.
Later came Blu-ray. The data here was even smaller and
therefore required an even shorter wavelength which
required a new colour. No prizes for guessing the
colour, the clue is in the name, it is of course a blue
laser. Each different colour requires a different
compound make-up in the semi-conductor material that
Data on CD as seen under the Scanning Electron
makes up the laser diode.
Microscope at IT Sligo
3D scanner using Laser spot triangulation
Two object positions are shown
In a 3D laser scanner the triangulation laser
shines a laser on the subject and uses a camera
to look for the location of the laser dot.
Depending on how far away the laser strikes a
surface, the laser dot appears at different places
in the camera's field of view. This technique is
called triangulation because the laser dot, the
camera and the laser emitter form a triangle.
If we look at the triangle made up by ABC, the
details we know are the angle formed by <CAB
and the distance between A and B. By taking the
point on the camera sensor we can draw a line
from there through the centre of the lens, this will
give us the angle formed <ABC. If you know
these three pieces of information about a triangle
then you can calculate the rest. Note in this
exampl the grey triangle appears to be a right angle
triangle but this will not always be the case, in the
case of the dashed line leading to the higher object
this angle is not a right angle.
In most cases a laser stripe, instead of a single
laser dot, is swept across the object to speed up
the acquisition process.
3D Scanner using Triangulation
Image author: Georg Wiora
Laser Printing.
Laser printing is a multipart process, but only one part of this process involves a laser.
First a rotating drum made from an appropriate material, usually selenium or an organic
photoconductor, is given a negative charge. A negative charge is created when an atom has
more electrons that protons. This is achieved by removing energy from the atom and allowing
an extra electron to fall into an orbit.
Now if a laser is shone very precisely at parts of the rotating drum, its photons will give energy
to these extra electrons and allow them to once again escape. This will mean that the atom is
once again neutral. The atoms that have been selected by the laser, together, form the image
that is to be printed. Next the toner, which is made of fine particles of dry plastic powder mixed
with carbon black or coloring agents, is given a negative charge. The rotating drum is then rolled in the
toner. Now most of the drum is negatively charged (the parts that do not form what we want to print).
These negative charges will repel the negatively charged toner, even though they are rolled across it,
because like charges repel each other. The parts that were made neutral by the laser however have no
such repulsion and in fact will attract the negative toner particles, although because they are neutral the
attraction is not very strong.
Now that the particles are on the drum, the drum is rolled across the paper where heat (up to 200º C)
and pressure are used to bond the toner particles to the paper. In some high end machines a negatively
charged roller is used on the back side of the paper to help pull the toner off of the drum onto the
paper.
Laser Cutting
The parallel rays of coherent light
from the laser source often fall in
range between 1.5875mm to
12.7mm in diameter. This is too
wide to achieve the localised
energy required to cut material.
Therefore this beam is normally
focused and intensified by a lens
mirror to a very small spot of
about 0.0254mm to create a very
intense laser beam.
the
or a
There are many different methods
in
Laser light shining on a material to be cut.
cutting using lasers, with different
types used to cut different material. Some of the methods are vaporization, melt and blow (Fusion),
reactive , thermal stress cracking, scribing, cold cutting and burning stabilized laser cutting.
Vaporization, Melt and blow, Reactive, Thermal stress cracking are described in brief below.
Vaporisation cutting
This method is normally used for non-melting
material such as wood, carbon and thermoset
plastics.
In vaporization cutting the focused beam heats
the surface of the material to boiling point and
generates a keyhole. Before the keyhole is formed
the surface reflects a lot of the light energy. The
creation of the keyhole leads to a sudden increase
in absorptivity quickly deepening the hole. As the
Laser Vaporisation cutting
hole deepens and the material boils, the vapor generated erodes the molten walls blowing ejecta out and
further enlarging the hole.
Melt and blow (also known as Laser Fusion Cutting)
Melt and blow or fusion cutting uses high-pressure
gas to blow molten material from the cutting area,
greatly decreasing the power requirement. In the case
of higher alloyed steels and aluminum, an inert gas
(nitrogen, argon) is typically used as a cutting gas.
Laser fusion cutting affords oxygen-free cut edges (no
oxidation), which is particularly important when
welding is the next process step after cutting. Today,
laser fusion cutting is used industrially for material
thicknesses of up to 15 mm.
First the material is heated to melting point then a gas
jet blows the molten material out of the kerf (up to
20 bar) avoiding the need to raise the temperature of
the material any further. Materials cut with this
process are usually metals.
Laser melt and blow, or fusion cutting.
Reactive cutting (also known as flame cutting or laser oxygen cutting)
Laser Reactive cutting is like oxygen torch cutting but with a laser beam as the ignition source.
The exothermic reaction of oxygen with the material (mainly steel) supports the laser cutting process
by providing additional heat input. It is mostly used for cutting carbon steel in thicknesses over 1mm.
This process can be used to cut very thick steel plates with relatively little laser power. The result is
higher cutting speed compared to laser fusion cutting with inert gases.
Thermal stress cracking
Brittle materials such as glass or acrylic are particularly sensitive to thermal fracture. This sensitivity is
used in thermal stress cracking. A Laser beam is focused on the surface of the material causing
localised heating and thermal expansion at a very precise point. This results in a crack that can be
grown and guided by moving the beam in the desired direction. The crack can be progressed relatively
quickly, in order of meters per second.